FIG. 1 illustrates a prior art PIN diode 10. PIN diode 10 includes an anode that is connected to an upper anode contact 12 and to a lower anode contact 12′, a cathode that is connected to a cathode contact 11, a semiconductor portion 20 that has a sensing region and an insulator 13 that is arranged to insulate the cathode contact from the upper anode contact 12. The insulator 13 and cathode contact 11 contact the upper surface of the semiconductor portion 20.
Upper anode contact 12 contacts the upper surface of the semiconductor portion 20 while the lower anode contact 12′ contacts the lower surface of the semiconductor portion 20—especially it contacts the lower surface of the substrate 24.
The semiconductor portion 20 includes a negative doped (n-type) cathode layer 21, an intrinsic layer 22, a positive doped (p-type) anode layer 23, a highly positive doped (P+ type) substrate 24, a highly negative doped (N+ type) region 25 and a highly positive doped (P+ type) region 26. FIG. 1 illustrates the n-type cathode layer 21 as being coated with a layer 27.
An upper surface of the n-type layer 21 is positioned below the layer 27 and defines the upper surface of a sensing region of the PIN diode 10. The sensing region may also include portions of the intrinsic layer 22 and the p-type layer 23.
The cathode contact 11 and the upper anode contact 12 are coupled to the N+ type and P+ type regions 25 and 26 respectively.
The n-type layer 21 is the cathode and it is coupled to the cathode contact 11 via the N+ type region 25.
The p-type layer 23 is the anode. The anode is coupled to the upper anode contact 12 via the P+ type region 26 and is coupled to the lower anode contact 12′ via substrate 24. Insulator 13 is positioned between the upper anode contact 12 and cathode contact 11. The N+ type region 25 may surround the n-type layer 21 and may also contact the intrinsic layer 22 and the p-type layer 23.
When PIN diode 10 is used as electron detector it is aimed to detect primary electrons (PE) that propagate in a vacuumed environment towards the PIN diode. Arrow 30 illustrates a primary electron that impinges on layer 27.
When the primary electron hits the active area (sensing region) of the PIN diode 10 it may create an electron\hole pair in the PIN junction formed by layers 21, 22 and 23.
The PIN diode 10 is biased by a reversed bias. Due to the applied reversed bias the electrons\holes pairs flow in the PIN junction and create a current signal through the PIN diode 10.
An inevitable byproduct of the primary electron interaction with the PIN diode 10 is the emission of electrons such as secondary electrons and backscattered electrons from the PIN diode 10 to the vacuum. The emitted electrons can interact again with various areas of the PIN diode—including the sensing region (see arrow 31) or with insulator 13 (see arrow 32). The trajectory of the emitted electrons in the vacuumed environment is a function of the electric field in the vicinity of the front surface of the PIN diode, the energy and exit angle of the emitted electrons.
The impingement of the emitted electrons on the PIN diode can cause unwanted results.
If, the emitted electron impinges on the insulator 13 it may charge the insulator 13 positively or negatively (depends on the energy of the emitted electron) and can create undesired electric potential on the insulator 13. This electric potential may remain even after the signal is ended, due to the very low charge mobility in insulator 13. This electric potential may act as a gating potential on the PIN diode 10 and may open a new current channel between 25 and 26 in parallel to the PIN junction. This undesired conductance channel will create a new parasitic dark current which will be added to the signal that flows in the junction. The parasitic dark current can be extremely large and might change the output signal of the PIN diode 10 dramatically.
There is a growing need to provide a sensing element with lower dark current.